Radio Frequency Identification (“RFID”) identifies and tracks products, animals, or people by transmitting and receiving radio frequency (“RF”) waves. Devices called RFID labels or tags are applied to or incorporated into the tracked object. Most RFID tags contain at least an antenna for receiving and transmitting the signal and an integrated circuit for storing and processing information, modulating and demodulating the RF signal, and other specialized functions. An RFID reader is used to “read”, i.e., wirelessly transfer, the information in these RFID tags for use with a variety of applications.
For example, RFID techniques may be used in all areas of an enterprise supply chain, e.g., manufacturers, distributors, wholesalers, and retailers, to automatically track and manage inventories. Because several individual RFID labels can reside in the receiving field of an RFID reader's antenna and be individually recognized, data may be transferred to and from each label individually or an entire electronic inventory may be obtained of the labels residing in the antenna field for a single RFID reader. One such application uses an RFID shelf reader to achieve this result. An RFID shelf reader typically includes an antenna or antenna array which is located in radiating proximity to, e.g., above or below, an inventory storage area, such as a shelf or cabinet. The RFID shelf reader can automatically detect and “read” each RFID label in the storage area.
In certain applications, RFID shelf reader antennas operate only in the near field or possibly in the radiating near field. An exemplary application is reading RFID labels affixed to DVDs. Additional description of such an exemplary use can be found in pending U.S. patent application Ser. No. 11/829,315, entitled, “RFID System with Integrated Switched Antenna Array and Multiplexer Electronics,” the entirety of which is incorporated herein by reference. Even if the DVDs are stacked together, the RFID labels are only a short distance away from the reader antenna and each label is located in a known position, i.e., either the bottom of the inside DVD cover or perhaps on the inside spine region. In other applications, such as for apparel, the RFID labels may be placed so that they reside in the near field, radiating near field, and the far field regions.
For shelf reader applications, where items with RFID tags attached are stacked on top of a shelf region, the orientation and location of the RFID tags may be such that the orientation may be random and the tag locations may reside in regions comprising the near field, radiating near field, and far field regions.
Prior RFID shelf reader implementations have used either a linear polarized antenna or a circularly polarized antenna. If the RFID label orientation is controlled and consistently in one particular direction, the linear polarized antenna solution is generally used. An example of a known RFID shelf reader linearly polarized patch antenna 10 is shown in FIG. 1. The linearly polarized patch antenna 10 typically includes a metal patch 12 suspended over a ground plane 14. The patch antenna 10 may be constructed using a dielectric substrate 16 to separate the metal patch 12 from the ground plane 14. For example, the patch antenna 10 may be constructed in the form of a printed circuit board, where one side includes the metal patch 12 and the opposite side includes the ground plane 14. The metal patch 12 may be fed, or activated, using a microstrip transmission line 18 as a feed line. Conventional linear polarized antennas radiate in only one direction, creating a null field in the orthogonal direction. Thus only one orientation of the RFID labels is covered. If an RFID label is misplaced or the item is improperly shelved, it will not be detected with a linear polarized antenna.
If the RFID label orientation is not well controlled, a circularly polarized antenna is usually preferred. An example of a known RFID shelf reader circularly polarized patch antenna 20 is shown in FIG. 2. The circularly polarized patch antenna 20 includes a circular or elliptical shaped metal patch 22 suspended over a ground plane 24 by a standoff 26 through the center, which grounds the patch 22 to the ground plane 24 below. For the circularly polarized patch antenna 20 of FIG. 2, the metal patch 22 is separated from the ground plane 24 by an air gap; however, the patch antenna 20 may be constructed using a dielectric substrate to separate the metal patch 22 from the ground plane 24. The circular or elliptical patch 22 is directly fed off center by element 28. The circularly polarized patch antenna 20 is designed to produce equal orthogonal electric field components in the far field.
The metal patch 22 is fed, or activated, using a feedpoint 28. A tuning stub 30 may be used to capacitively tune the resonating frequency of the patch antenna 20. The tuning stub 30 is a metal cylinder whose vertical position in relation to the metal patch 22 is adjustable. FIG. 3 provides a side view of the circularly polarized patch antenna 20 illustrating the positioning of the tuning stub 30.
The radiating pattern of the circularly polarized antenna changes orientation with the phase of the excitation signal so all orientations of the RFID label may be detected. However, while the field orientation for the linear polarized antenna is very consistent from the near field to the far field distances away from the antenna, this is not the case for the simple circularly polarized antenna. Circularly polarized antennas have some regions of very weak fields especially in the near field or radiating near field. The near field and radiating near field exhibit null zones for certain angles of the RFID tag for the circularly polarized antenna and only in the far field is the behavior of the field omni-directional and free from null zones.
Additionally, other devices may also use a patch antenna to communicate with a large number of devices at one time. One such device is the Electronic Article Surveillance (“EAS”) tag deactivation system described in U.S. patent application Ser. No. 12/331,604, entitled “Metal Oxide Semiconductor Device for Use in UHF EAS Systems,” the entire teachings of which are hereby incorporated by reference. As with the RFID shelf reader described above, the antenna of the EAS deactivation system must be in relatively close proximity to EAS tags in order to transmit a signal capable of deactivating the EAS tags. When deactivating a large number of EAS tags simultaneously, it is important that the EAS tag orientation is known and controlled, or the radiating field must be uniform and omni-directional. Otherwise, not all intended EAS tags will be deactivated. As shown above, known antenna configurations are not reliable for this use.
In either case, the fields above the surface of these antennas have null zones or weak field zones at either the near field, the radiating near field, or the far field. These approaches only work in specific applications and circumstances.
Therefore, what is needed is a system that provides an antenna solution having a field that is well behaved in the near field, the radiating near field, and the far field in all possible label or tag orientations above the antenna.